No textures for Smooth Terrain in Thumbnails

Pretty much just water appears black and everything else is white, like for the first few seconds you convert to smooth terrain in studio.

Place: Smooth Terrain Valley - Roblox

Yup, this will be fixed next week. If you want a workaround, put a file mesh in your place (e.g. insert a part, put a SpecialMesh there and for id use rbxasset://fonts/head.mesh)